1. Field of the Invention
This invention relates to the polymerization of elastomeric isoolefinic homopolymers and copolymers, especially the polymerization reaction required to produce the isobutylene-isoprene form of butyl rubber. More particularly, the invention relates to an improved method of stabilizing against agglomeration the polymerization slurries used in the preparation of such polymers; the medium, or diluent of such slurries being methyl chloride or certain other polar chlorinated hydrocarbon diluents.
2. Prior Art
The term "butyl rubber" as used in the specification and claims means copolymers of C.sub.4 -C.sub.7 isoolefins and C.sub.4 -C.sub.14 conjugated dienes which comprise about 0.5 to about 15 mole percent conjugated diene and about 85 to 99.5 mole percent isoolefin. Illustrative examples of the isoolefins which may be used in the preparation of butyl rubber are isobutylene, 2-methyl-1-propene, 3-methyl-1-butene, 4-methyl-1-pentene and .beta.-pinene. Illustative examples of conjugated dienes which may be used in the preparation of butyl rubber are isoprene, butadiene, 2,3-dimethyl butadiene, piperylene, 2,5-dimethylhexa-2,4-diene, cyclopentadiene, cyclohexadiene and methylcyclopentadiene. The preparation of butyl rubber is described in U.S. Pat. No. 2,356,128 and is further described in an article by R. M. Thomas et al. in Industrial and Engineering Chemistry, vol. 32, pp. 1283 et seq., October, 1940. Butyl rubber generally has a viscosity average molecular weight between about 100,000 to about 800,000, preferably about 250,000 to about 600,000 and a Wijs Iodine No. of about 0.5 to 50, preferably 1 to 20.
The term isoolefin homopolymers or polymers as used herein is meant to encompass those homopolymers of C.sub.4 -C.sub.7 isoolefins particularly polyisobutylene, which have a small degree of terminal unsaturation and certain elastomeric properties.
The principal commercial forms of these butyl rubber and isoolefin polymers such as isobutylene-isoprene butyl rubber and polyisobutylene, are prepared in a low temperature cationic polymerization process using Lewis acid type catalysts, typically aluminum chloride being employed. Boron trifluoride is also considered useful in these processes. The process extensively used in industry employs methyl chloride as the diluent for the reaction mixture at very low temperatures, that is less than minus 90.degree. C. Methyl chloride is employed for a variety of reasons, including the fact that it is a solvent for the monomers and aluminum chloride catalyst and a nonsolvent for the polymer product. Also, methyl chloride has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers.
The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 30% by weight in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. Also, an acceptable, relatively low, viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
Notwithstanding the widespread use of the slurry polymerization process in methyl chloride, there are a number of problems in carrying out this process which are related to the tendency of the polymer product particles to agglomerate, and thereby destabilize the slurry dispersion. In the absence of special additives, the rate of agglomeration increases rapidly as reaction temperature approaches -90.degree. C. and it is not possible to maintain a stable slurry above -80.degree. C. These agglomerated particles tend to adhere to, grow and plate out on all surfaces they contact, such as reactor discharge lines, as well as reactor inlet lines and any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.
One technique used by industry to circumvent this difficulty has been operation of the reactor below -80.degree. C. and with high agitation. It has become standard practice to design manufacturing facilities with additional reactor equipment so that the reaction process can be cycled between alternate reactor systems so that at any given time one or more reactors are in the process of being cleaned or adhered polymer. If a stable slurry is produced and maintained in a non-fouling condition, substantial economies in equipment installation and process techniques are achieved.
A general reference text which discusses the theory and principles concerning dispersion polymerization and in particular the use of block and graft copolymers as dispersion stabilizers is "Dispersion Polymerization in Organic Media", edited by K. E. J. Barrett, John Wiley & Sons, 1975. While this text, particularly in Chapter 3, discloses the use of block or graft copolymers having an insoluble component, or anchor group, and a diluent-soluble component in a number of dispersion polymerization processes, no disclosure is made of any stabilizer system useful in the methyl chloride slurry polymerization process for isoolefin homopolymers or butyl rubber copolymers as disclosed in accordance with the present invention.
In published Netherlands Application 770760 (1977), filed in the U.S. on June 14, 1976, as Ser. No. 699,300, Markle et al disclose a non-aqueous dispersion polymerization process for conjugated diolefins in the presence of a block copolymer dispersion stabilizer, at least one block being soluble in the liquid organic dispersion medium and at least another block being insoluble in the dispersion medium. The Markle et al disclosure deals with the polymerization of a conjugated diolefin monomer in a liquid hydrocarbon dispersion medium such as n-butane, neopentane or mixed isomeric pentanes in the presence of a Ziegler-Natta Catalyst. The conjugated diolefins, particularly preferred by Markle et at, are butadiene-1,3, isoprene and piperylene. Markle et al also disclose mixtures of conjugated diolefins.
The process of the present invention is considered distinguished from the disclosure of Markle et al in that it relates to a cationic polymerization carried out in a polar chlorinated hydrocarbon diluent, such as methyl chloride, utilizing stabilizers which are especially effective in that polymerization process. Markle et al deal with coordination polymerization processes conducted in a nonpolar liquid hydrocarbon diluent.
An effective method for stabilizing methyl chloride slurries, or slurries in various polymerization diluents used in the production of isoolefin polymer products, using chemical additive stabilizers is disclosed by K. W. Powers and R. H. Schatz in U.S. Pat. No. 4,252,710; the invention disclosed herein is an improvement over that earlier teaching. The '710 reference discloses that such slurries can be stabilized through the addition of minor proportions of a preformed copolymer stabilizer or an in situ formed stabilizer. The stabilizer is generally described as having a lyophilic, diluent soluble portion and a lyophobic, diluent insoluble, isoolefin homopolymer or butyl rubber soluble or adsorbable portion; in the case of an in situ formed stabilizing agent, the stabilizer precursor was disclosed as a lyophilic polymer containing a functional group capable of copolymerizing or forming a chemical bond with the isoolefin polymer or butyl rubber copolymer being formed in the polymerization process, the latter polymer or copolymer being the lyophobic portion of the stabilizing agent.
The '710 reference discloses that certain categories of preformed stabilizers, while effective as slurry stabilizers are preferably added upon completion of polymerization because they contain substantial amounts of cationically active unsaturation or functional groups. A continuation-in-part patent to the same inventors, U.S. Pat. No. 4,358,560, based on application Ser. No. 236,719, further defines the effect of such cationic activity on the use of preformed stabilizers. The invention disclosed herein is a further improvement over the earlier teachings of Powers and Schatz in the '560 reference as well.
The '710 and '560 patents broadly disclose "hydrogenated diene polymers, e.g. hydrogenated polybutadiene" (column 5, lines 15-16) as suitable lyophobic materials; claim 8 in each patent reflects that language as well. Examples 2(b) and 3 of the '710 patent demonstrate the limitations of certain of the stabilizers in which cationically active isoprene moieties are present. These patent references represent a starting point for the comprehensive and specific discoveries disclosed herein.